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Adding interior insulation on a brick veneer house - Concern about inward solar drive caused moisture issues.

I own a 1929 brick veneer home and have been steadily improving the airtightness and insulation levels. The time has come to consider the exterior walls, which are (from inside to outside) plaster over celotex, 2x4 framing (hybrid balloon framing), ship lap sheathing, tar paper, 3/4" air gap, then brick. The house is 2 stories, and has approximately 2' overhangs. The walls currently have about 1" of insulation in the form of some sort of prairie grass encapsulated in kraft paper, which is nailed in place with 1/4 wood strips.

In an ideal world, we'd tear out the exterior walls, insulate full depth with closed cell spray foam, and then re-install drywall, perhaps with a layer of rigid foam under the drywall to reduce thermal bridging and bring the wall thickness up at least to the original thickness so that baseboards and such can be re-fitted.

Should I be concerned about potential wetting of the sheathing and framing? I'm in a mixed/cold climate in south east MN.

Alternately, I'd also consider low expanding injection foam (though I haven't found a contractor I'd trust yet) or blown cellulose installed from the inside.

The alternate approaches have appeal because of the cost savings and because the existing plaster is in great shape.

Asked by josh_in_mn
Posted May 20, 2014 8:36 AM ET


12 Answers

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Answered by Acobo
Posted May 20, 2014 9:27 AM ET


Have you read this article? Insulating Old Brick Buildings

Answered by user-756436
Posted May 20, 2014 9:58 AM ET


I've read the paper and the article. Both address solid masonry walls. I've got a veneer wall with a ventilation space vented to the soffit behind the brick.

Answered by josh_in_mn
Posted May 20, 2014 10:14 AM ET


Sorry for sending you to the wrong article. The air gap and the asphalt felt go a long way toward reducing the risk of inward solar vapor drive, especially if the air gap has openings at the base of the wall and the top of the wall to encourage air flow. If the air gap is clogged with mortar droppings, and it doesn't get much air flow, that's another story.

Celotex Corporation manufactured many products over the years, including polyisocyanurate (rigid foam) and fiberboard sheathing. I'm not sure which Celotex product you are referring to.

Answered by user-756436
Posted May 20, 2014 12:28 PM ET


The Celotex I'm speaking of is a pressed board plaster backer product. It's got keys molded into one side, and is relatively smooth on the other. Strangely, this house has the celotex backer on outside walls, and on the upstairs ceiling, but traditional lath everywhere else. I can only think they were shooting for better air-tightness back in the day.

Where we've done some demo for an addition, we saw that the gap wasn't all that plugged up with mortar droppings. The brickwork is in really nice shape with very few cracks or other problems.

So, would you say that if I created some weep holes at the bottom of the wall, I should be pretty safe if I wanted to install a non-vapor permeable insulation in the stud cavities? Also what about the idea of a thermal break on the inside? Would I risk problems with the framing then?

Answered by josh_in_mn
Posted May 20, 2014 12:42 PM ET


Q. "Would you say that if I created some weep holes at the bottom of the wall, I should be pretty safe if I wanted to install a non-vapor permeable insulation in the stud cavities?"

A. If you are worried about inward solar vapor drive, non-vapor-permeable insulation will cause fewer problems than vapor-permeable insulation.

Answered by user-756436
Posted May 20, 2014 1:04 PM ET



Is there any research that indicates the likelihood of experiencing inward solar drive related problems in a situation as I've described? I'd really rather have the superior air tightness and R-value afforded by closed cell insulation if I can do it without rotting my house.

I'm kind of inclined to go ahead with the closed cell plan, partly because I'm not sure how I'd contain blown insulation from going into the space between floors, given that this house is balloon framed, and does not have any blocking in the wall as near as I can figure.

Answered by josh_in_mn
Posted May 20, 2014 1:09 PM ET


The classic solar vapor drive problem involves a "reservoir" cladding (you have that), no air space or an air space that is compromised by mortar droppings (you don't have that), vapor-permeable sheathing (you have that), fluffy insulation that is both air-permeable and vapor-permeable, interior polyethylene, and air conditioning.

If you don't have interior polyethylene, I wouldn't worry.

Obviously, if you can find a way to install spray foam or injection foam, you won't have any problems.

Answered by user-756436
Posted May 20, 2014 1:15 PM ET


"In an ideal world, we'd tear out the exterior walls, insulate full depth with closed cell spray foam, and then re-install drywall, perhaps with a layer of rigid foam under the drywall to reduce thermal bridging and bring the wall thickness up at least to the original thickness so that baseboards and such can be re-fitted."

Are you going to gut the interior finish wall?

There's nothing ideal about full depth closed cell foam. In fact the best you can do is about 1/4-1/2" shy of full depth because the stuff is so hard to trim flush with the studs, which means you end up with slightly more severe thermal bridging. Even at a theoretical full-fill the thermal bridging robs closed cell foam of nearly half it's performance potential, even in a balloon framed structure (about a 20% framing fraction typical. for 16" o.c. construction sans firestop blocking). Save the high R/inch goods for the thermal break layer where you reap the full value.

Then there's the matter of the extremely high lifecycle global warming of closed cell foam blown with HFC245fa (almost all R6/inch or better foams used in the US are blown with it), at about 1000x CO2.

And at 3+" the stuff has a permeance of less than 0.4 perms, which means there is effectively zero potential for drying toward the interior.

If you gut the wall you'd be fine with 1" of closed cell on the interior (about 1 perm) as an air seal and semi-permable condensing surface, with the rest of the cavity filled with damp spray cellulose, or compressed batts, if you can be obsessive about perfect fit, rock wool preferred. Many 1920s homes were framed with full-dimension 2x4s that run between 3.8-4.2" in actual depth, which means even with 1" foam against the ship-lap you don't have to compress it much, but there may be non-standard width issues to work around too, which is why sprayed/blown products are a better choice. With ~R6 outside of ~R10-R12 of fiber you have a sufficient foam/fiber ratio for dew point control without needing to go low-permeance on the interior side- standard latex paint is good enough.

If you're not going to gut it, blowing it full of cellulose at 3-3.5lbs density works. If the latter it's advisable in your climate zone to reduce the vapor retardency of the interior finish to something between 0.5-1 perms, which can be done with "vapor barrier latex" paint (about 0.5perms).

In a full-gut situation you could cut in some 1.5" wide strips of 1/2"foil-faced polyiso glued to the framing edges as a thermal break before completing the cavity fill. If the center cavity depth from wallboard to your spray foam is more than ~3" nominal with the strips added it makes the center cavity R ratio a bit marginal, and a "smart" vapor redarder such as Certainteed MemBrain or Intello Plus would be called for. If you have the room for it go ahead and add more. With polyiso, at 3/4" you would be roughly doubling the R-value at the framing. You could also go for full sheets of polyiso on the interior,caulked to the framing and seams sealed with FSK tape and skip the smart vapor retarder. That would yield a modest improvement in whole-wall-R, but it would also eliminate any possibility of drying toward the interior, with a modest risk of summertime condensation inside the wall cavity. (That risk is pretty low if you vent the masonry cavity both top & bottom, and have 1-perm spray foam on the ship-lap.)

Answered by user-1004076
Posted May 20, 2014 1:23 PM ET
Edited May 20, 2014 1:24 PM ET.



You've given me a bunch to think about. Thanks. I'm now leaning towards either the full sheets of polyiso or perhaps something like what I've shown in the attached image:

The advantage of the scheme shown in the image is that the spray foam could lap onto the foam "caps" over the studs and form a continuous layer. I have enough room to add 1" or more of foam on the interior without problems.

Answered by josh_in_mn
Posted May 22, 2014 9:49 AM ET
Edited May 22, 2014 9:51 AM ET.


If you take the approach as shown in the sketch, use open cell foam trimmed flush with the caps, not closed cell, with vapor barrier latex on the wallboard. The cost will be much lower, the air tightness comparable or better, the difference in whole-wall R would be negligible, the greenhouse gas footprint about 10,000x lower, and you could skip the side-cap detail (just use 1.5" end caps glued/tacked in place prior to foaming.)

If you went with full sheet iso instead of caps you can skip the v.b. latex, and the open cell foam would be easier to trim (otherwise you'd be nicking your end caps if your not careful.)

Answered by user-1004076
Posted May 22, 2014 1:58 PM ET


I'm in roughly the same boat as Joshua, but in Climate 5B (cold-dry Denver, also not sure how big or open the air-space behind the brick is, I don't have any weep holes, so I still have some investigating to do.)

If foam strips are added to the interior studs as you suggested, will 1" of CC spray foam against the sheathing with the balance of the space filled with damp-spray cellulose or fiberglass work?


Answered by Ericwest1
Posted Jul 17, 2014 4:37 PM ET

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